I never had to buy one separately, I have one from my old iPhone which is for German sockets and one from my new one which is for UK sockets, but u mostly charge my phone through computer anyway.
I have never bought an Apple charger. Still using the one that came with my iPhone 4S and I bought it in December 2011. The iPod that I bought before that (in 2010 or so) came with a charger too, and that is still working.